1. Field of the Invention
The present invention relates, in general, to fatigue damage indicators and, more particularly, to a multidirectional fatigue damage indicator, which is capable of monitoring the actual degree of fatigue damage for a structure under complex fatigue loads.
2. Description of the Prior Art
For the purpose of monitoring the fatigue damage of a structure under complex fatigue loads, the fatigue monitoring devices such as fatigue fuse and fatigue indicator have been proposed. Those conventional devices are usually attached along the direction of the maximum principal stress after the direction is determined. Therefore, the conventional fatigue monitoring devices are actually capable of monitoring fatigue damage only in one fixed direction. However, the directions and the magnitudes of the principal stresses, in general, change during transient loading condition.
The other important characteristics of fatigue is that the actual direction of fatigue damage does not always coincide with the direction of maximum principal stresses because fatigue damage depends upon the number of occurrences as well as the magnitude of the stresses. For example, consider the two transient states, one with large magnitude of stress and small number of occurrences, and the other with the smaller magnitude but the larger number of occurrences. In this case, the actual fatigue damage can occur along the direction of the smaller principal stress if the number of occurrence is sufficiently larger. Therefore, it is not reasonable to monitor the fatigue damage in one pre-determined direction because this may underestimate the actual damage.
In brief, the conventional fatigue monitoring devices can not predict reasonably the actual progress and the location of fatigue damage because the monitoring is limited to only one direction. In addition, they usually require burdensome estimation of the principal stress direction before attaching them to the structure.
The present fatigue damage indicator has been invented to resolve the above problems occurring in the prior art. The objective of the present invention is to provide more reliable fatigue damage indicator which is capable of monitoring the actual progress of fatigue damage in any direction and eliminating burdensome process of determining the maximum principal stress direction prior to attachment to the structure under combined fatigue loads.
In order to accomplish the above objective, the present invention provides a multidirectional fatigue damage indicator, composing of a series of fuse elements in circumferencial direction to cover the changing characteristics of the principal stresses direction. In addition, the semicircular rows of the fuse elements are arranged in radial direction to monitor the progress of the fatigue damage by watching the sequential fracture of the fuse elements in a radial direction. The fuse elements in each row have the same fatigue lifetime with the same crack lengths but the lifetimes are different from row to row with different crack lengths.
In addition, the present invention provides another shape of multidirectional fatigue damage indicator, composing of two rows of semicircular fatigue fuse elements in full circular assembly frame to monitor the degree of fatigue damage in alternative way. Another merit of the present invention is that this indicator has no limit in the number of fuse elements such as even or odd numbers and the rows of the fuse elements can be added according to the required reliability for the fatigue damage indicator.